Optical waveguides are well known in the art, and devices incorporating optical waveguides have been employed in many different fields as communicators, sensors and monitors. An optical waveguide typically consists of a dielectric core fabricated from a material having a certain refractive index, surrounded by a second material having a lower refractive index. This surrounding material is generally known as the cladding. A beam of light is guided by this composite structure as long as the refractive index of the core material exceeds the refractive index of the cladding material. A light beam within the core is guided generally along the core axis by reflection at the boundary between the core and cladding.
A number of different designs for optical waveguides have been developed including the multimode step index profile, the single mode step index profile, and the multimode graded index profile. Where single mode transmission is desired, the single mode optical waveguide is used. In such a waveguide, the diameter of the core is typically less than 10 .mu.m and the difference between the refractive indices of the cores and the cladding is on the order of 10.sup.-3 to 10.sup.-2. At wavelengths which are longer than a critical wavelength, called the LP.sub.11 cutoff wavelength, only the lowest order optical mode will be supported in such a waveguide.
Optical fibers have also been fabricated which include multiple cores disposed in numerous different arrays and positioned within a common cladding. One such disclosure is contained in U.S. Pat. No. 4,148,560. This disclosure is directed toward an assembly including a plurality of fibers embedded in an encapsulating material. This particular patent shows an optical bundle positioned between two reinforcing wires and embedded in a protective sheath of plastic material.
The phenomenon known as cross-talk between cores in a common cladding occurs when the light energy propagating along one core is coupled to an adjacent core. This occurs because, as is known, the propagating optical energy is not totally confined by the boundary between the core and cladding but, in fact, it penetrates to a degree into the cladding.
U.S. Pat. No. 4,295,738 discloses a pressure sensitive optical waveguide comprising multiple, nonconcentric cores formed in a manner that cross-talk between adjacent cores is primarily a function of the strain or hydrostatic pressure applied to the waveguide.
Also known are multiple clad fibers such as those disclosed in U.S. Pat. No. 4,435,040; however, all of the concentric, guiding layers of this and similar patents are configured to interact with the mode field diameter of the optical power distribution passing through the unperturbed fiber. For this reason, these waveguides cannot be utilized for detecting pressure variations on the fiber. The intent and purpose of such multiply clad fibers is to alter the optical dispersion characteristics of the waveguide. Optical dispersion affects the information carrying capacity or bandwidth of a fiber.